1. Field of the Invention
The present invention relates to a device and method for measuring distance using acoustic waves.
2. Description of the Prior Art
It is known in the art to determine the distance between a location and an object from the time (t) taken for an acoustic wave generated at the location to be reflected from the object and return to the location. If the velocity (v) of the acoustic wave is known then the distance (s) from the location to the object can be calculated using the equation s=v×t. This equation is idealistic. In practice, propagation of the acoustic wave is effected by temperature and humidity of the air through which it travels. Typically a 10 degrees Celsius (degC) change in temperature results in 2 percent change in velocity which translates into a 2 percent error in distance measurement. Therefore, there is a need to compensate the affects of temperature and humidity.
A device for determining distance using sound waves is described in U.S. Pat. No. 4,464,738, but the device ignores the effects of temperature and humidity on acoustic wave propagation.
U.S. Pat. No. 4,581,726 discloses an ultrasonic distance measuring device that compensates for variation of propagation characteristic of the ultrasonic wave. However, it compensates for the effect of temperature and humidity on the attenuation of wave propagation in the air and not the temperature dependency of sound velocity, which also significantly affects the accuracy of distance measurement.
U.S. Pat. No. 4,910,717 discloses a distance measuring device in which a charging circuit for a timing capacitor has two temperature dependent diodes. The diodes compensate for the effect of temperature on velocity of the sound wave. However, there would be difficulty in selecting and matching diodes with suitable accuracy. Furthermore, temperature change may not be as simple as a step change, and complicated temperature changes may give an imprecise result of temperature estimation.
U.S. Pat. No. 5,442,592 discloses a distance meter that cancels out the effects of temperature and humidity variations by including a measuring unit and a reference unit having a known distance. However, a practical actualization of the idea in measuring a physical distance up to 20 meters with a portable device is virtually infeasible.